Makes 6 sandwiches
1/2 cup olive oil
6 tablespoons white vinegar
1 3/4 teaspoons dry mustard, divided
1 tablespoon dark brown sugar
1 2-pound well-trimmed tri-tip beef roast
4 large plum tomatoes, very thinly sliced
1 bunch watercress, stems trimmed
1/2 white onion, very thinly sliced
6 5-inch pieces French baguette, cut horizontally almost in half, opened flat
Prepare barbecue (medium-high heat). Whisk oil, vinegar, and 3/4 teaspoon mustard in large bowl. Season dressing with salt and pepper. Transfer 2 tablespoons dressing to small bowl; whisk in sugar and 1 teaspoon mustard. Sprinkle beef all over with salt and pepper, then top with mixture from small bowl.
Grill beef until thermometer inserted into thickest part registers 120°F to 125°F for rare, turning occasionally, about 30 minutes. Place on cutting board; let rest 10 minutes. Cut very thinly across grain.
Meanwhile, add tomatoes, watercress, and onion to dressing in large bowl. Marinate while beef grills, tossing occasionally. Grill bread, cut side down, until just crusty, about 3 minutes; place on plates, grilled side up.
Arrange beef slices over bread. Using tongs and draining well, top with salad.